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Iraqis celebrate the return by U.S. forces of cities and towns to Iraqi control (Reuters video).

WASHINGTON (Reuters) -- U.S. President Barack Obama has lauded the U.S. troop withdrawal from towns and cities in Iraq as an important milestone but warned of "difficult days" ahead.

Obama said U.S. forces in Iraq had met a June 30 deadline to complete their withdrawal from urban areas and hand over full control to Iraqi security forces.

All U.S. troops are due to withdraw from Iraq by 2012, nine years after invading to topple Saddam Hussein.

It was a day of celebration in Iraq as U.S. forces handed control of the cities to Iraqi authorities.

But the top U.S. commander was less than joyous when pressed on how many of his troops would remain.

Speaking via satellite from Baghdad, U.S. Army General Ray Odierno lost his cool at a briefing for Pentagon reporters when he was repeatedly questioned about the number of U.S. troops that would remain in the cities as advisers to Iraqi forces.

Asked why he could not give a figure, he became visibly irritated, raised his voice, and replied: "Because it would be inaccurate! Because I don't know exactly how many are in the cities. It varies day-to-day based on the mission."

Pressed to give a rough figure, he snapped: "How many times you want me to say that? I don't know."

Odierno, one of the most formidable figures in the U.S. military, apologized for his outburst at the end of the briefing.

"Sorry I lost my temper a little bit on the number," he said, to some laughter from reporters.